1. Field of Invention
This present invention generally relates to cerebrospinal fluid (CSF) shunts and, more particular, to a device and method for testing for the presence, absence and/or rate of CSF flow through shunt tubing implanted under the skin in hydrocephalus patients.
2. Description of Related Art
Hydrocephalus is a condition of abnormal cerebrospinal fluid (CSF) homeostasis, resulting in an accumulation of CSF in the brain ventricles. Approximately 69,000 people are diagnosed with hydrocephalus each year in the United States, most commonly as a congenital condition, making it one of the most common birth defects. Untreated hydrocephalus leads to progressive neurological dysfunction and death.
The most commonly used treatment for hydrocephalus is diversion of CSF from the ventricles to the peritoneal cavity by means of a permanent prosthetic shunt. A CSF shunt is comprised of a valve connected to a tube. The proximal end of the tube is surgically inserted into the ventricle of the brain, and runs subcutaneously through the body into the abdominal cavity (FIG. 1). There are approximately 300,000 shunted hydrocephalus patients in the US. 41,000 shunt procedures are performed each year, approximately 12,000 of which are new shunt placements.
Improved materials, diagnostic, and treatment technologies, have improved shunt therapy since the 1970s. However, shunt failure is still almost inevitable during a patient's life. The one-year failure rate of ventriculoperitoneal shunts has been estimated to be approximately 40%, and the mean period to failure of an implanted shunt is typically only 5-10 years. Obstruction of the ventricular catheter, usually from tissue ingrowth or clots, is overwhelmingly the greatest cause of shunt failure. Shunt failure can rapidly progress to life-threatening elevation in intracranial pressure, so revision surgery, and re-placement of the blocked ventricular catheter is indicated. More than half of all shunt procedures in the United States are revisions.
However, since catheter replacement surgery carries risks of life-threatening complications such as infection or embolism, a need for shunt revision needs to be reasonably established. The problem is that the usual clinical manifestations of shunt failure, such as headaches, vomiting, or loss of vision, are non specific and are difficult to differentiate from common, less serious illnesses, particularly in pediatric patients. This leads to two extremes of management: patient families who present persistently at emergency rooms for every headache or flu symptom, and patient families who dangerously dismiss symptoms of a shunt blockage as a common ailment. A study at the Children's Hospital of Philadelphia (CHOP) indicates that they see three false alarms for every true shunt malfunction. There is a need for objective methods to evaluate suspected shunt obstruction.
An unacceptably high number of hydrocephalic children still die as a result of shunt malfunction, primarily because of a failure to identify shunt blockage at an early stage. The early diagnosis of shunt obstruction is complex and difficult. While a number of shunt flow detection methods are available, none are diagnostic when used alone or are without complication, and there is little standardization to guide physicians in their interpretation (Table 1). Physical examination of the patient, including pumping of the shunt reservoir, is unreliable. Measuring CSF pressure by “shunt tap” is invasive, painful, and can be misleading. CT and MR, either alone, or in combination with plain radiographs, remain the gold standards for diagnosis of shunt malfunction. However, these imaging techniques are static, and so must be performed multiple times to detect ventricular enlargement. This results in repeated radiological exposures of patients (often children), a safety concern for pediatric neurosurgeons. Furthermore, the reliability of these techniques for detecting CSF accumulation has been questioned. For a while, radionuclide markers were widely used to derive truly dynamic information about CSF flow in the brain and in shunts. However, their promise was never wholly realized, and they are not routinely utilized in most clinical settings. Because of the expense and technical complexity of advanced imaging techniques, they cannot be used to investigate every headache.
TABLE 1Table 1: Performance of Commonly-Used DiagnosticProcedures for Suspected CSF Shunt ObstructionSensitivitySpecificityDiagnostic(Detecting(DetectingProcedureNo Flow)Flow)FeaturesStatic Imaging ProceduresCT Scan68%90%Expensive, time-consuming,[36]radiation dose. Shunt malfunctionmust have gone on long enough forthe scan to detect visible changes, i.e.ventricle enlargement. Risingconcern about radiation.X-ray27%99%Expensive and time-consuming. AsSerieswith CT, the shunt must have[36]malfunctioned long enough forvisible changes to be detected.Dynamic Flow MeasurementsShunt Tap79%56%Method is painful, risks infection and[37]can be inconclusive if blockage isupstream of the tap area.Radio80%53%Requires an invasive shunt tap andIsotope24 hours lead time for isotope. This[38]method is considerably moreinvolved than either the CT or MRI.
The current, non-invasive imaging procedures have relatively low sensitivity and better specificity—making them reasonable rule-in tests but poor rule-out tests. The invasive procedures are somewhat better rule-out tests, but are painful and present an infection risk. Furthermore, children are often sent to CT Scans, the most commonly used procedures, when they present to the ER and such repeat exposure to radiation may be harmful. What is needed is a simple and reliable method for determining CSF shunt flow rates that can be interpreted by neurosurgeons and non-neurosurgeons with equal confidence.
To meet the need for rapid and sensitive methods for determining shunt function, Applicant has developed a method to allow non-invasive detection of cerebrospinal fluid flow through subcutaneous shunts under the rubric “ShuntCheck.” ShuntCheck involves devices using thermal dilution technology—detecting a transcutaneous change in temperature as cooled cerebrospinal fluid flows through the subcutaneous portion of a ventriculoperitoneal shunt. See U.S. Patent Application No. 2013/0109998 (which is incorporated by reference in its entirety herein), owned by the same Applicant as the present application, namely, ShuntCheck, Inc. As shown most clearly in FIG. 2A, one early version 10 of ShuntCheck, namely, “ShuntCheck v2.2”, comprises a single use disposable thermosensor 12 which is placed on the skin 11 over a subcutaneous shunt 13; a personal digital assistant (PDA)-based BioDisplay 14 which includes an A/D converter for converting and conditioning the analog sensor signal into digital signal; this PDA-based BioDisplay includes ShuntCheck software which analyzes temperature data from the thermosensor and provides a time-temperature graph and a flow or no-flow result. Side 12A of the thermosensor 12 faces upward when placed against the skin 11 and side 12B (FIG. 2B) is adhesively secured to the skin 11, once a release strip 15 is removed. ShuntCheck also includes the “Micro-Pumper” (see FIGS. 2C and 2D), a device which generates a temporary increase in CSF flow in patent, but not in occluded CSF, shunts. See U.S. Patent Publication No. 2013/0102951, also owned by ShuntCheck, Inc. and which is also incorporated by reference in its entirety herein.
As shown in FIG. 2D, the Micro-Pumper 300 is hand-held device that is positioned against the skin 11 over the dome portion 210 of the CSF shunt's valve 211; the valve 211 is typically implanted over bone 220 of the patient's skull. A foot 301 having short rods 302 is reciprocated against the dome 210 by a Micro-Pumper drive system (FIG. 2C) including a shaft 306, a spring 305, a piston 304 and a cam 307 driven by a motor (not shown).
As shown in FIG. 2B, the thermosensor 12 comprises a plurality of temperature sensors TS (e.g., thermistors, e.g., GE thermistors, by way of example only) and is adhesively placed on the skin 11 where the shunt 13 crosses the clavicle. Ice 17 (or, e.g., ice within a receptacle) is placed on the skin, “upstream” of the CSF flow (viz., in window 19) from the plurality of temperature sensors 17, to cool the CSF in the shunt 11. Temperature sensors TS placed over the shunt 13 detect the change in temperature as cooled fluid flows beneath them. The presence of flowing fluid is interpreted as a decrease in temperature detected by the temperature sensors TS, while no change in temperature indicates the absence of flow. The ShuntCheck method/devices can assess the rate of CSF fluid flow through shunts. The temperature drop recorded by ShuntCheck method/devices varies linearly with flow rate-the deeper the temperature drop, the faster the flow (see FIG. 3).
Further testing of the thermosensor 12 determined that intermittent CSF flow is likely to be a limiting factor on specificity performance of any method in which shunt patency or obstruction is being inferred from fluid flow measurements. As a result, the Micro Pumper was developed. As discuss previously, the Micro Pumper 300 is a miniature, non-invasive device which is held against the shunt valve (which is typically implanted under the scalp behind the ear) and which provides a specific vibration pulse to the valve. The vibration pulses act like a manual shunt pumping in miniature and generate a temporary increase in shunt flow through patent, but not through occluded shunts. However, in certain instances flows enhanced by use of the Micro Pumper 300 resulted in flows beyond the detection of the ShuntCheck v 2.2 device.
Thus, there remains a need for an improved thermosensor design and a new method for providing thermal dilution cooling which permits the non-invasive detection of cerebrospinal fluid flow through subcutaneous shunts.
All references cited herein are incorporated herein by reference in their entireties.